Liquid-liquid extractions are often used in chemical processes to transfer a solute dissolved in a first liquid to a second liquid which is essentially immiscible with the first liquid. The solution of the solute in the first liquid is generally termed a feed solution and the second liquid is generally termed an extractant. The undissolved solute can be a solid, a liquid, or a gas. When the feed solution is brought into contact with the liquid extractant, the solute tends to distribute itself between the two liquids in accordance with the relative solubility of the solute in the two liquids. Since the feed solution and the liquid extractant are essentially immiscible, they form two distinct thermodynamic phases when in contact with one another. These two phases can be physically separated from one another, which effects a separation of a fraction of the solute from the feed solution.
In order to promote a rapid distribution of the solute between a feed solution and an extractant in conventional liquid-liquid extraction processes, the feed solution and the extractant are typically mixed together intimately. However, such intimate mixing frequently gives rise to troublesome problems. For example, the mixing generally involves forming a dispersion of one of (the liquids in the other. Frequently, the resulting dispersion is relatively stable, so that it takes a long time for the dispersed liquid to coalesce. As a result, the throughput of the solute-transfer process is undesirably low or the inventory of feed solution and extractant tied up in the process is undesirably high.
U.S. Pat. No. 3,956,112 to Lee et al. refers to an extraction process in which a porous membrane serves as a partition between two immiscible solvents. Solutes from one solvent are transferred to the other solvent via direct solvent-solvent contact by way of the porous membrane. In practice, however, conventional extraction processes in which immiscible solvents are separated by a porous membrane generally do not prevent one solvent from forming a dispersion in the other. Typically one or the other solvent seeps through the porous membrane and becomes dispersed in the solvent on the other side of the membrane. As a result, conventional extraction processes involving immiscible solvents separated by a porous membrane generally must provide a settling tank and a solvent return mechanism to coalesce the dispersion formed by the seepage of one of the solvents through the membrane and to return the solvent thus recovered to its source.